The present invention relates to a laryngeal mask airway device. More specifically, the present invention relates to an improved airway tube for use with such devices.
The laryngeal mask airway device is a well known device that is useful for establishing airways in unconscious patients. Such devices have been in use for about thirteen years and offer an alternative to the older, even better known, endotracheal tube. For at least seventy years, endotracheal tubes comprising a long slender tube with an inflatable balloon disposed at the tube's distal end have been used for establishing airways in unconscious patients. In operation, the endotracheal tube's distal end is inserted through the mouth of the patient, past the patient's laryngeal inlet (or glottic opening), and into the patient's trachea. Once so positioned, the balloon is inflated so as to form a seal with the interior lining of the trachea. After this seal is established, positive pressure may be applied to the tube's proximal end to ventilate the patient's lungs. Also, the seal between the balloon and the inner lining of the trachea protects the lungs from aspiration (e.g., the seal prevents material regurgitated from the stomach from being aspirated into the patient's lungs).
Although they have been enormously successful, endotracheal tubes suffer from several major disadvantages. The principal disadvantage of the endotracheal tube relates to the difficulty of properly inserting the tube. Inserting an endotracheal tube into a patient is a procedure that requires a high degree of skill. Also, even for skilled practitioners, insertion of an endotracheal tube is sometimes difficult or not possible. In many instances, the difficulty of inserting endotracheal tubes has tragically led to the death of a patient because it was not possible to establish an airway in the patient with sufficient rapidity.
In addition to this principal disadvantage, there are also other disadvantages associated with endotracheal tubes. For example, intubation with an endotracheal tube often causes patients to suffer from severe “sore throats.” The “sore throat” is principally caused by friction between the tube and the notch between the patient's arytenoid cartilages. Another disadvantage is that patients can not cough effectively while intubated with an endotracheal tube. Yet another problem with endotracheal tubes relates to the manner in which they are inserted. Inserting an endotracheal tube normally requires manipulations of the patient's head and neck and further requires the patient's jaw to be forcibly opened widely. These necessary manipulations make it difficult, or undesirable, to insert an endotracheal tube into a patient who may be suffering from a neck injury. Still another disadvantage is that endotracheal tubes provide an airway that is relatively small or narrow. The size of the airway must be relatively narrow because the distal end of the tube must be sufficiently small to fit into the trachea.
In contrast to the endotracheal tube, it is relatively easy to insert a laryngeal mask airway device into a patient and thereby establish an airway. Also, the laryngeal mask airway device is a “forgiving” device in that even if it is inserted improperly, it still tends to establish an airway. Accordingly, the laryngeal mask airway device is often thought of as a “life saving” device. Also, the laryngeal mask airway device may be inserted with only relatively minor manipulations of the patient's head, neck, and jaw. Further, the laryngeal mask airway device provides for ventilation of the patient's lungs without requiring contact with the sensitive inner lining of the trachea and the size of the airway established is typically significantly larger than the size of the airway established with an endotracheal tube. Also, the laryngeal mask airway device does not interfere with coughing to the same extent as endotracheal tubes. Largely due to these advantages, the laryngeal mask airway device has enjoyed increasing popularity over the last thirteen years.
FIGS. 1A and 1B show perspective and side views, respectively, of a prior art laryngeal mask airway device 100. FIG. 2 illustrates a device 100 that has been inserted into a patient. Laryngeal mask airway devices such as device 100 are described for example in U.S. Pat. No. 4,509,514. Laryngeal mask airway devices similar to device 100 have been marketed commercially as the “Classic” for many years by the Laryngeal Mask Company of Cyprus. Device 100 includes a flexible cylindrical airway tube 110 and a mask portion 130. Tube 110 extends from a proximal end 112 to a distal end 114 and mask portion 130 is coupled to the tube's distal end 114. Mask portion 130 includes a proximal end 132 and a generally elliptical inflatable cuff 134. Mask portion 130 also defines a central passageway extending from proximal end 132 to an open end 136 of cuff 134. The distal end 114 of tube 110 fits telescopically into the cylindrically shaped proximal end 132 of mask portion 130, and device 100 provides a continuous, sealed, airway extending from proximal end 112 of tube 110 to the open end 136 of cuff 134. Device 100 also includes an inflation tube 138 for selectively inflating or deflating cuff 134.
In operation, the cuff 134 is deflated, and then the mask portion is inserted through the patient's mouth into the patient's pharynx. The mask portion is preferably positioned so that a distal end 140 of cuff 134 rests against the patient's normally closed esophagus and so that the open end 136 of the cuff 134 is aligned with the entryway of the patient's trachea (i.e., the patient's glottic opening). After the mask portion is so positioned, the cuff is inflated thereby forming a seal around the patient's glottic opening and this establishes a sealed airway extending from the proximal end 112 of the tube 110 to the patient's trachea.
For convenience of exposition, the term “fully inserted configuration” shall be used herein to refer to a laryngeal mask airway device that has been inserted into a patient and has the following characteristics: (1) the mask portion is disposed around the patient's glottic opening; (2) the cuff is inflated forming a seal around the patient's glottic opening; and (3) the airway tube extends from a proximal end located outside the patient's mouth to a distal end that is coupled to the mask portion, the tube extending through the patient's mouth and the patient's natural upper airway so that the device provides a sealed airway extending from the tube's proximal end to the patient's lungs. FIG. 2 shows a laryngeal mask airway device in the fully inserted configuration.
When device 100 is in the fully inserted configuration, device 100 advantageously does not contact the interior lining of the trachea. Rather, the seal is established by contact between the tissues surrounding the patient's laryngeal inlet and the inflatable cuff 134. Unlike the delicate interior lining of the trachea, the tissues at the laryngeal inlet are accustomed to contact with foreign matter. For example, during the act of swallowing food, the food is normally squeezed against these tissues on its way to the esophagus. These tissues are accordingly less sensitive and less susceptible to being damaged by contact with the inflatable cuff.
U.S. Pat. No. 5,303,697 describes an example of another type of prior art device that may be referred to as an “intubating laryngeal mask airway device.” The intubating device is useful for facilitating insertion of an endotracheal tube. After an intubating laryngeal mask airway device has been located in the fully inserted configuration, the device can act as a guide for a subsequently inserted endotracheal tube. Use of the laryngeal mask airway device in this fashion facilitates what is commonly known as “blind insertion” of the endotracheal tube. Only minor movements of the patient's head, neck, and jaw are required to insert the intubating laryngeal mask airway device. Once the device has been located in the fully inserted configuration, the endotracheal tube may be inserted with virtually no additional movements of the patient simply by inserting the endotracheal tube through the airway tube of the intubating laryngeal mask airway device. This stands in contrast to the relatively large motions of the patient's head, neck, and jaw that would be required if the endotracheal tube were inserted without the assistance of the intubating laryngeal mask airway device.
As shown in FIG. 2, when the device 100 is in the fully inserted configuration, the airway tube 110 assumes a curved profile that is principally defined by the shape of the patient's natural upper airway (i.e., the patient's natural airway passage defined by anatomical structures such as the hard and soft palettes and the pharynx that allows air to freely pass between the mouth and the glottic opening). For convenience of exposition, the term “inserted shape” will be used herein to refer to the shape assumed by the airway tube when a laryngeal mask airway device is in the fully inserted configuration and the terms “resting shape” or “resting configuration” will be used herein to refer to the shape assumed by the airway tube when no external forces are acting on the tube (e.g., when the device is not inserted into a patient and is simply at rest).
In intubating laryngeal masks, the airway tube is often formed from rigid or semi-rigid material and the tube's resting shape is often identical or nearly identical to the tube's inserted shape. However, it is not always desirable to form the airway tube out of rigid material. For example, use of rigid materials, such as metal, for the airway tube increases the cost of the device and may also complicate insertion of the device.
Other laryngeal mask airway devices, such as device 100, use a more flexible airway tube for which the resting shape is different, and substantially straighter, than the tube's inserted shape. Use of such flexible airway tubes can facilitate insertion, and reduce the cost, of the device. However, it also requires the airway tube to bend or flex during insertion and to remain in a flexed, or stressed, position for as long as the device remains in the patient. In device 100, the amount by which the airway tube flexes during insertion (or the difference between the tube's resting and inserted shapes) is reduced by manufacturing the tube such that its resting shape is slightly curved rather than straight. FIG. 1B shows the resting shape of the airway tube 110 of device 100.
Several factors influence the design of the airway tube for a flexible tube device such as device 100. The airway tube 110 should be sufficiently flexible to permit the tube to easily flex or bend between the resting and inserted shapes. However, the tube 110 should also be sufficiently stiff, or have sufficient strength, to resist formation of kinks when flexing to the internal shape. FIG. 3 shows an example of a tube that has formed a kink 180 as a result of bending the tube by an extreme amount. As is well known, the size of the internal passageway defined by any tube is dramatically decreased at any such kinks 180. The effects of kinks in tubes is commonly experienced in connection with garden hoses. For example, formation of a single kink in a garden hose can dramatically decrease the amount of water that can pass through the hose and be distributed by a sprinkler The effects of kinks are similar in laryngeal mask airway devices. Any kinks forming in the airway tube of a laryngeal mask airway device essentially close off the tube's airway passage and dramatically decrease the volume of air that can pass through the tube. Accordingly, it is very important to design the airway tube so that kinks in the tube do not form when the tube flexes to the internal shape. The tube should be flexible enough to permit relatively easy movement between the resting and internal shapes, but not so flexible as to cause formation of kinks when the tube is flexed to the internal shape.
Device 100 achieves this compromise with a generally cylindrical airway tube 110. If not for the pre-bend (shown in FIG. 1B) of the airway tube that makes the tube's central axis curved rather than straight when the tube is in its resting position, the tube would be essentially perfectly cylindrical. FIG. 4 shows a cross section of airway tube 110 taken along line 4-4 as shown in FIG. 1B. As shown in FIG. 4, the outer perimeter 110-o of the airway tube 110 is circular. The inner perimeter 110-i of tube 110, which defines the tube's internal airway passage, is also circular. Also, the inner and outer perimeters, 110-i and 110-o, respectively, are centered about a common point C. The airway tube 110 may be fabricated from poly vinyl chloride (PVC) or silicone characterized by a durometer of about 50-80 Shore A. In adult male sizes, the inner radius (i.e., the distance from the center C to the inner perimeter 110-i) is substantially equal to 5 millimeters and the outer radius Ro, (i.e., the distance from the center C to the outer perimeter 110-o) is substantially equal to 7.5 millimeters, such that the thickness T of the wall of the airway tube 110 is substantially equal to 2.5 millimeters.
Although the airway tube 110 of device 100 achieves the desired compromise of being sufficiently flexible to permit easy insertion into a patient (and relatively easy flexing between the resting and internal shapes) and sufficiently stiff to prevent formation of kinks when bent to the inserted shape, the tube 110 is under stress whenever it is in the internal shape. This stress reflects the resilient airway tube's tendency to automatically return to its resting shape. As a result of this stress, a force F, as shown in FIG. 2, is applied to anatomical structures of the patient whenever the device 100 is in the fully inserted configuration.
There remains a need for improved airway tubes for use with laryngeal mask airway devices.